In the field of mobile communications, a multi-station system is used for, for example, radio paging and an automobile telephone, because the multi-station system eliminates the necessity of handover, which is required when a user moves from one zone to the next, and uses less transmission power, thereby allowing a large service area to be covered. In recent years, the multi-station system has been expected to be applied to a road-to-vehicle communication system. The multi-station system comprises a plurality of radio base stations forming small radio zones in which the same data is transmitted almost concurrently to or received from each of the small radio zones with the same frequency, thereby forming one large service area.
As shown in FIG. 19, a conventional mobile communication system, to which the multi-station system is applied, comprises a communication control station 1100, a plurality of radio base stations 1200-1 to 1200-n and a mobile station 1300. The communication control station 1100 is positioned at some distance from each of the plurality of radio base stations 1200-1 to 1200-n. The plurality of radio base stations are connected to the communication control station 1100 by inter-station transmission paths 1400-1 to 1400-n, respectively. A base station includes the communication control station 1100, the plurality of radio base stations 1200-1 to 1200-n and the inter-station transmission paths 1400-1 to 1400-n, and the base station performs communications with the mobile station 1300. In such a system, in order to monitor the remote radio base stations 1200-1 to 1200-n from the communication control station 1100, each of the radio base stations 1200-1 to 1200-n is required to transmit, to the communication control station 1100, monitoring data indicating a state of the radio base station such as whether the radio base station is properly working, other than data received from the mobile station 1300.
In general, the conventional mobile communication system adopts a method in which wired cables such as fiber optics are used for the inter-station transmission paths 1400-1 to 1400-n, and a general-purpose interface is used for the communication control station 1100 and the radio base station 1200, thereby realizing inter-station data transmission with a transmission rate higher than that of a radio link (refer to non-patent documents 1 and 2). FIG. 20 shows an exemplary structure of a base station of a conventional mobile communication system to which the multi-station system is applied. FIG. 21 shows a detailed exemplary structure of a transmission path interface unit 1210 of FIG. 20. FIG. 22 shows a detailed exemplary structure of a transmission path interface unit 1120 of FIG. 20.
The communication control station 1100 includes the control unit 1110 and the transmission path interface unit 1120. The transmission path interface unit 1120 includes a data buffer 1121, a protocol control unit 1122, a clock signal source 1123, a PHY interface (I/F) unit 1124 and an optical media converter 1125. Each of the radio base stations 1200-1 to 1200-n includes the transmission path interface unit 1210 and a radio unit 1220. The transmission path interface unit 1210 includes an optical media converter 1211, a first PHY interface unit 1212, a first protocol control unit 1213, a downlink data buffer 1214, a first clock signal source 1215, a second clock signal source 1216, an uplink data buffer 1217, a second protocol control unit 1218 and a second PHY interface unit 1219.
On a downlink from the communication control station 1100 to the mobile station 1300, data such as transmission data directed to the mobile station 1300 and control data directed to the radio base station 1200 are transmitted from the control unit 1110 to the transmission path interface unit 1120. These data received at the transmission path interface unit 1120 are temporarily stored into the data buffer 1121, and sequentially converted, by the protocol control unit 1122, the clock signal source 1123, the PHY interface unit 1124 and the optical media converter 1125, into a transmission format that is based on a specification of a general-purpose interface (e.g., 100BASE-FX). The resultant data is then transmitted, with a transmission rate higher than that of a radio link, to each of the radio base stations 1200-1 to 1200-n via the inter-station transmission paths 1400-1 to 1400-n, respectively.
On the other hand, on an uplink from the mobile station 1300 to the communication control station 1100, a radio signal received at the radio unit 1220 from the mobile station 1300 is detected and modulated, and then converted into a digital baseband signal. The digital baseband signal is then transmitted, as reception data, to the transmission path interface unit 1210. Monitoring data is also transmitted from the radio unit 1220 to the transmission path interface unit 1210. The reception data and the monitoring data received at the transmission path interface unit 1210 are sequentially converted, in the same manner as that of the downlink, into the transmission format that is based on the specification of the general-purpose interface, and then transmitted, with the transmission rate higher than that of the radio link, from the radio base stations 1200-1 to 1200-n to the communication control station 1100 via inter-station transmission paths 1400-1 to 1400-n, respectively.
Thus, by performing the inter-station transmission between the communication control station 1100 and the radio base stations 1200-1 to 1200-n with the transmission rate higher than that of a radio link by using the general-purpose interface, it becomes possible to provide, other than a range for transmitting reception data from the mobile station 1300, an available range into which monitoring data can be inserted for transmission. Non-patent document 1: “Development of New DSRC Wireless Equipment complying with ITS”, Hitachi Kokusai Denki Gihou, No. 3, pp. 10-22. Non-patent document 2: “W-CDMA radio base station device”, Matsushita Technical Journal, Vol. 47, No. 6, pp. 10-22.